1. Field of the Invention
This invention relates to an apparatus and process for delivering a vapor phase product having a constant impurity level from a liquefied gas source to an end point. More specifically, this invention relates to an apparatus and process that delivers a liquefied gas having a level of soluble impurities from a storage container to a vaporization unit external from the storage container, whereupon the liquefied gas and the soluble impurities are completely converted to the vapor phase having a substantially equivalent (i.e., constant) concentration of said impurities as in the liquefied gas, thereby preventing the build-up of such impurities in the storage container or in the vaporization unit. The vapor phase product is then directed to a point of use such as a semiconductor fabrication tool.
2. Description of the State of Art
Electronic specialty gases (ESG""s) play an important role in the production of integrated circuits. Examples of such specialty gases include ammonia (NH3), hydrogen chloride (HCl), hydrogen bromide (HBr), chlorine (Cl2), tungsten hexafluoride (WF6), hydrogen fluoride (HF), carbon dioxide (CO2), nitrous oxide (N2O), dicholorosilane (SiH2Cl2), phosphine (PH3), arsine (AsH3), silane (SiH4), disilane (Si2H6), chlorine trifluoride (ClF3), and boron trichloride (BCl3). Additional ESG""s include, for example, the class of materials known as perfluorocarbons (PFC""s). Processes for the production of integrated circuits using such gases include chemical vapor deposition (CVD), diffusion, reactive ion etching (RIE), plasma and thermal etching of silicon and gallium arsenide wafer surfaces, deposition of silicon nitride layers, metal organic chemical vapor deposition (MOCVD) and growth of gallium nitride films in light emitting diodes (LED""s). Moisture or any other impurities in the electronic specialty gases can adversely affect the performance of all these processes. These impurities can be carried to the semiconductor fabrication tools used in these processes and consequently have a direct impact on the wafer yield.
Typically, these electronic specialty gases are stored in a storage container as compressed gases in the liquid phase under their own vapor pressure, and are used in a semiconductor fabrication tool in the vapor phase. Currently, conventional gas delivery systems require the gas to be vaporized directly within the storage container and then delivered throughout the gas distribution system. However, conventional gas phase delivery systems possess many problems due to the inability to maintain constant flow for long periods of time. Additionally, gasses delivered from conventional gas delivery systems have inconsistent impurity concentrations. The build-up of impurities within conventional gas delivery systems causes severe fluctuations in the impurity concentrations as a function of time, temperature, pressure and flow rate. These fluctuations may influence the process performance, and the build-up of impurities may reduce the amount of product usage. Furthermore, it is impossible to have a specification for a certain impurity on a fall storage container, since the concentrations of impurities change as the container is consumed.
Conventional gas delivery systems using cylinders also require frequent cylinder changes and multiple connections, which increase the probability of impurity build-up within the container, decrease the lifetime of gas manifolds, and increase the chance for accidents. Delivery systems which do not require frequent cylinder changes and which are capable of high flow rates greater than 5000 standard liters per minute will soon be required by the semiconductor industry because of the transition to larger fabrications along with introduction of 300 mm wafers. Furthermore, conventional gas delivery systems can cause the formation of liquid droplets that are entrained in the gas flowing through the gas delivery system. These entrained droplets contain contaminants that increase corrosion and lead to the failure of downstream components such as regulators, valves, mass flow controllers, and pressure transducers. Moisture in corrosive gases can farther lead to metallic particulate contamination within the gas distribution system, which has a direct impact on the wafer yield.
U.S. Pat. No. 5,644,921 discloses an apparatus for storing ultra-high purity non-cryogenic liquefied compressed gases and a method for delivering a vaporized gaseous product produced from the liquefied gas for semiconductor process applications. The delivery method includes withdrawing and heating a gaseous product from a storage vessel containing the liquefied compressed gas, then piping the heated gas through the liquid contained in the storage vessel in a heat exchange fashion.
U.S. Pat. No. 6,032,438 describes a method and a system for the delivery of a vapor phase product to a point of use and an on-site chemical distribution system and method. The system includes a storage vessel containing a liquid chemical under its own vapor pressure, a column connected to receive the chemical in a liquefied state from the storage vessel where the chemical is fractionated into a contaminated liquid heavy fraction and a purified light vapor fraction, and a conduit connected to the column for removing the purified light vapor fraction therefrom. This system requires that the residual contaminated liquid be periodically drained.
U.S. Pat. No. 5,894,742 describes a method and system for delivering an ultra-high purity gas to a point of use. The method involves transporting an ultra-high purity pressurized liquid from a container to a phase change device (i.e., an evaporator) where the pressurized liquid-phase gas is converted into the gas phase, and delivering the gas to a point of use. The flow of liquid into the evaporator is controlled by sensors that maintain the liquid level in the evaporator to about 70% of total capacity. In this type of evaporator, the impurities dissolved in the liquid phase build up in the evaporator, similar to conventional gas phase delivery systems. The phase change device of this system does not allow for 100% vaporization since a pool of liquefied gas constantly resides in the evaporator. This system also requires the whole system, including the bulk container, the distribution conduits, and the evaporators to be totally emptied of its liquid content periodically. The entire system must then be carefully cleaned and purged before being refilled with fresh product. The purpose of this periodical maintenance is to discard the growingly impure liquid phase chemical from the system. Thus, the build up of the soluble impurities and subsequent elimination of a portion of the liquid phase chemical ultimately causes the consumer to discard 10-30% of the original amount of the liquid chemical.
Conventional vaporizers, which are used for the vaporization of liquefied gases in the semiconductor industry, may be classified as follows: (i) a heating medium separated from the evaporating liquid by tubular heating surfaces, (ii) a heating medium confined by coils, jackets, double walls, flat plates, etc., (iii) a heating medium brought into direct contact with evaporating liquid, and (iv) heating by solar radiation (see, Perry and Green, Chemical Engineer""s Handbook, 1984). Delivery systems that allow the evaporating liquid to be stored in a storage tank, where the evaporating liquid is not effectively in contact with the heating medium, work by principles very similar to a conventional gas delivery system. The liquefied gas undergoes a single plate distillation within the vaporizing device and is only partially vaporized. Therefore, this kind of vaporizer causes the concentration of impurities to change with time. Conventional vaporizers, which are used for the vaporization of liquefied gases in the semiconductor industry, are of this kind, i.e., they only partially vaporize the liquefied gas. Therefore, with conventional vaporizers, it is difficult to deliver the gas with a constant concentration of impurities.
The semiconductor industry is currently requiring higher vapor phase flow rates with lower and more consistent impurity levels. However, conventional gas delivery systems cannot maintain sustained flow rates above 900 standard liters per minute (slpm). There is still a need, therefore, for a gas delivery system that is capable of maintaining constant impurity levels throughout the consumption of a storage container and is able to maintain high flow rates (e.g., greater than 1000 slpm) for long periods of time. There is also a need for a system that is able to deliver the entire contents of the storage container. Such a system would not only reduce the costs associated with the material and reduce the frequent replacement of the storage container, but also would avoid the extra costs related to the recovery or disposal procedures of the waste gas.
To meet the stringent requirements of the semiconductor industry and to overcome the problems of conventional gas delivery systems, this invention provides a unique system for the delivery of a gas, such as an electronic specialty gas, from a liquid source. The delivery system of this invention prevents the buildup of impurities, such as water or any other impurities dissolved in liquid phase of the stored gas, both within the storage container and the vaporization unit. The delivery system of this invention is therefore able to maintain substantially constant levels of the soluble impurities in the both the liquid phase and the vapor phase of the gas throughout the delivery process, thereby leading to constant yields in the production of end products utilizing gases from the gas delivery system of this invention.
Accordingly, one embodiment of this invention provides a delivery system for supplying a vapor phase product having substantially constant impurity levels from an initially liquefied form of a gas from at least one storage container to an endpoint, comprising:
a vaporization unit comprising an inlet, an outlet, a vaporizing means for converting a liquefied gas having a concentration of soluble impurities to the vapor phase, and a heating means for heating said vaporizing means to a temperature sufficient to completely vaporize said liquefied gas and said soluble impurities, wherein said vaporizing means is capable of completely vaporizing said liquefied gas and said soluble impurities to form a vapor phase product before said liquefied gas and said soluble impurities accumulate in said vaporization unit, wherein said vapor phase product has an impurity level that is substantially equivalent to the impurity level of said impurities in said liquefied gas;
a first delivery conduit for delivering said liquefied gas from said at least one storage container to said vaporization unit; and
a second delivery conduit for delivering said vapor phase product from said vaporization unit to said endpoint.
By completely vaporizing all of the liquefied gas and the soluble impurities that enter the vaporization unit, a pool of liquid where the soluble impurities can build up does not accumulate in any component of the vaporization unit. Thus, the design of the delivery systems of this invention prevents buildup of the soluble impurities in both the liquefied gas source and the vaporization unit.
The delivery systems of this invention allow for use of substantially all of the contents of the storage container by ensuring that the concentration of soluble impurities within the storage container does not fluctuate throughout the consumption of contents of the container.
Further, the delivery systems of this invention maintain substantially constant impurity concentrations that are independent of the starting concentration of the impurities dissolved in the liquefied gas.
The delivery systems of this invention are also capable of maintaining high vapor phase flow rates for longer periods of time relative to conventional gas delivery systems. In addition, the delivery systems of this invention are also capable of maintaining constant impurity levels throughout the consumption of the contents of small containers (e.g., 10 milliliter bubblers) at low vapor phase flow rates for long periods of time. More specifically, the gas delivery systems of this invention are capable of maintaining constant impurity levels throughout the consumption of a storage container at vapor phase flow rates from about 0.001 standard liters per minute (slpm) to at least 10,000 slpm for long periods of time.
The delivery systems of this invention further allow the delivery of a vapor phase product to one or more endpoints without leading to the formation of liquid droplets which can cause fluctuations in impurity concentrations and/or increase corrosion and cause the failure of downstream components such as regulators, mass flow controllers, and pressure transducers.
Additional advantages and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The features and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods particularly pointed out in the appended claims.